(1) Field of the Invention
The present invention relates to heat-resisting phosphate based compound sintered bodies and production thereof. More particularly, the invention relates to phosphate based compound sintered bodies having excellent heat resistance and low expansion as well as excellent high temperature stability, and to a process for producing the same.
(2) Related Art Statement
With recent progress in industrial technologies, demands for materials having excellent heat resistance and low expansion have increase.
As a result of these demands, it has been realized that zirconyl phosphate [(ZrO).sub.2 P.sub.2 O.sub.7 ] is promising as a material having excellent heat resistance and low expansion.
Further, zirconium phosphates of alkali metals such as sodium have been proposed as materials having heat resistance and low coefficients of thermal expansion [Mat. Res. Bull., Vol. 19, pp. 1451.about.1456 (1984); Journal of Materials Science 16, 1633.about.1642 (1981); and Ceramics Association Report 95(5), pp.531 to 537(1987)].
Furthermore, with respect to calcium (Ca) among alkaline earth metals, calcium phosphate based compounds having special compositions have been proposed as materials with low expansion (Mat. Res. Bull., Vol. 20, pp 99.about.106, 1985).
However, although the above phosphate compounds such as zirconyl phosphate have a merit of excellent low expansion, these phosphate compounds have a common problem that they are thermally decomposed at high temperatures not lower than 1,200.degree. C. to evaporate a phosphorus component. For instance, when zirconyl phosphate is thermally treated at 1,400.degree. C. for 100 hours, it exhibits a weight reduction by as much as 19%.
On the other hand, in order to produce phosphate compounds, there are known processes using combinations of Na.sub.2 CO.sub.3, ZrO.sub.2, ZrOCl.sub.2.8H.sub.2 O, SiO.sub.2, (NH.sub.4).sub.2 HPO.sub.4, H.sub.3 PO.sub.4, Nb.sub.2 O.sub.5, Y.sub.2 O.sub.3, SrCO.sub.3, K.sub.2 CO.sub.3, CaCO.sub.3 and/or the like [T. Oota and I. Yamai, Journal of the American Ceramic Society, 69, 1, (1986)].
However, in the above producing processes, a P.sub.2O.sub.5 component is produced alone during decomposition of ammonium phosphate or H.sub.3 PO.sub.4 so that portions of a high concentration of phosphorus are locally formed to produce a low melting point compound during sintering. Consequently, large pores (spaces) are formed in the sintered bodies, thereby causing serious defects.